On different levels, if you look at the front or back of a hunters mid-section with the night vision of a sniper rifle, you can see that oval-like sections of the mid-section pulse. I think that it serves as some sort of circulatory/respiritory purpose, possibly breathing (as others have stated). But why does still pulse after they fall? Maybe the hunters don't really die from any known firepower, or maybe it serves as an automattic decomposision of the body after death.